This disclosure relates generally to a memory cell structure and layout for a phase change memory (PCM) comprising poly-emitter bipolar junction transistors (BJTs) as access devices.
There are two major groups in computer memory: non-volatile memory and volatile memory. Constant (or nearly constant) input of energy in order to retain information is not necessary in non-volatile memory but is required for volatile memory. Thus, non-volatile memory devices contain memory in which the state of the memory elements may be retained for days to decades without power consumption. Examples of non-volatile memory devices include Read Only Memory (ROM), Flash Electrical Erasable Read Only Memory, Ferroelectric Random Access Memory, Magnetic Random Access Memory (MRAM), and Phase Change Memory (PCM). Examples of volatile memory devices include Dynamic Random Access Memory (DRAM and Static Random Access Memory (SRAM).
In a PCM, information is stored in memory cells comprising phase change elements (PCEs). A PCE comprises materials that can be manipulated into different phases. Each of these phases exhibits different electrical properties that may be used for storing information. Amorphous and crystalline phases are two phases typically used for bit storage (1's and 0's), as they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase. An access device may supply the current necessary to change a PCE from one phase to another. Each PCE in the PCM may have a single associated access device.
A PCM array may be configured in a cross-point architecture, with the PCEs controlled by an access device such as a complementary metal oxide semiconductor (CMOS) transistors or diodes at every junction. However, a problem in high density PCM design is the drive current required to cause PCE phase changes. The small metal oxide semiconductor field effect transistor (MOSFET) devices necessary to provide a high density of bits per unit area may not provide sufficient current to switch the phase of a PCE. A tightly packed diode array may provide enough drive current to cause resistive phase change in a PCE, but a significant amount of current may cross over to adjacent PCEs, causing cross-talk between PCEs in high density PCMs. Another possibility is the use of bipolar junction transistors (BJTs) as access devices. However, integration of CMOS and BJT arrays has proven difficult; further discussion of this problem may be found in U.S. application Ser. No. 12/121,875 (Rajendran et al.), filed May 16, 2008, which is herein incorporated by reference in its entirety.